Author
Topic: What happens during electrolysis of salt water with copper electrodes? (Read 84877 times)

I have salty water which I perform electrolysis on using copper probes (cathode and anode). One of the copper probes nothing happens to as far as I can see (canít remember which one it is) but the other starts to wear away and turn the water blue/green.

From what I can tell this is copper chloride, but there is a slight brown substance (maybe just dirt off the coin).

What is created at the other probe?Sodium acid??For some reason I think I remember a teacher of mine saying adding acid to the water will help it on its way (this is why I added the salt) is this true as I have potassium acid and potassium hydroxide.

Is there a simple why of copper plating using electrolysis which is safe and doesnít use poisons?

Hi Jonathan, welcome to the forum and congrats on your amateur experimenting !

We've discussed this topic in some detail previously, but to summarise :

When you pass an electric current through a solution, ions (charged particles) migrate towards the electrode of the opposite charge.

In a salt solution (NaCl) the dominant species of ions are sodium (Na+) and chloride (Cl-), because only a tiny amount of water (H2O) is ionised (to H+ and OH-) at pH 7, and that's why pure water is very difficult to electrolyse (and why teachers add acid to help the process).

So when you apply a current to the solution using copper electrodes, the chloride ions (Cl-), termed anions, will move towards the positive electrode (the anode), whilst the positively-charged sodium ions (the cations) will migrate towards the negative electrode (the cathode). The migrating ions carry charge through the solution and hence help to complete the circuit.

At the anode 2 chloride ions (Cl-) will each surrender an electron to the anode (which likes electrons because it is positively charged) to form a molecule of chlorine gas, which you see fizzing off :

2Cl-(aq) -> Cl2(g) + 2e-

At the same time, the copper (Cu) forming the electrode will also try to donate electrons :

Cu(s) -> 2e- + Cu2+(aq). When the copper (Cu) gives up 2 electrons it forms a copper ion (Cu2+) which then goes into solution, turning the electrolyte blue / green, as you have observed.

At the negative electode (cathode) hydrogen ions (H+) from water pick up electrons to form hydrogen :

2H+(aq) + 2e- -> H2(g)

...and the copper ions (Cu2+) which were mobilised from the anode also pick up electrons to form metallic copper which is deposited on the cathode :

Cu2+(aq) + 2e- -> Cu(s).

So if you weighed the 2 electrodes before and after the experiment, and carried on for long enough, you should be able to demonstrate that the anode (the positive electrode) gets lighter, and the cathode (the negative electrode) gets heavier.

Chris

"I never forget a face, but in your case I'll make an exception" - Groucho Marx

You mentioned the safety of copper chloride. Here's a snippet from the Material Safety Data Sheet. Keep in mind that these things are usually a bit on the pessimistic side to figure for all possible reactions and health effects and usually account for large doses.

Potential Health Effects----------------------------------

Inhalation:Causes irritation to respiratory tract, symptoms may include coughing, sore throat, and shortness of breath. May result in ulceration and perforation of respiratory tract. When heated, this compound may give off copper fume, which can cause symptoms similar to the common cold, including chills and stuffiness of the head.Ingestion:May cause burning pain in the mouth, esophagus, and stomach. Hemorrhagic gastritis, nausea, vomiting, abdominal pain, metallic taste, and diarrhea may occur. If vomiting does not occur immediately systemic copper poisoning may occur. Symptoms may include capillary damage, headache, cold sweat, weak pulse, kidney and liver damage, central nervous excitation followed by depression, jaundice, convulsions, blood effects, paralysis and coma. Death may occur from shock or renal failure.Skin Contact:Causes irritation, redness, and pain. Some individuals may develop copper allergies.Eye Contact:Causes irritation, redness, pain, discoloration and damage. May cause conjunctivitis, ulceration or clouding of the cornea.Chronic Exposure:Prolonged or repeated skin exposure may cause dermatitis. Prolonged or repeated exposure to dusts of copper salts may cause discoloration of the skin or hair, blood and liver damage, ulceration and perforation of the nasal septum, runny nose, metallic taste, and atrophic changes and irritation of the mucous membranes.Aggravation of Pre-existing Conditions:Persons with pre-existing skin disorders, impaired liver, kidney, or pulmonary function, glucose 6-phosphate-dehydrogenase deficiency, or pre-existing Wilson's disease may be more susceptible to the effects of this material.

This message brought to you by The Council of People Who Are Sick of Seeing More People

Nope, you explained it as well as I could have. Well done. If you're really up for being bored, I can go into the physical chemistry of why this reaction happens, but I don't think either of you want me to go there.

This message brought to you by The Council of People Who Are Sick of Seeing More People

I was thinking about this last night (for some reason !) and thought up a question I couldn't answer.

If you use say an iron cathode to electrolyse a solution of copper sulphate, the iron cathode becomes 'copper plated'. What would happen, physically and chemically, if you instead used a carbon cathode ? That, presumably, would not become copper plated ?

Chris

"I never forget a face, but in your case I'll make an exception" - Groucho Marx

The carbon cathode would become copper plated, I did it once. It was in the Usbourne book of, probably, chemistry. You used a copper electrode at the anode and a leaf covered in graphite powder as the cathode in a copper sulphate solution. After a while the leaf got gilded in copper which was quite cool.

Would carbon deposit on copper? For a while, I was using carbon for a positive due to corrosion issues, but it turns out the carbon rods were fouled. They emit clouds of this strange blue goop, so I went back to copper. Would enough copper chlorides to turn the water brown, with black clouds affect the reaction in a good or bad way?

Ok im new to the forums and I am a highschool student working and a project that uses water electrolysis to make hydrogen for a alternate fuel source and i was wondering what exactly breaks down the water why does it happen? I have looked in the library and on the internet and can not find the exact reason on why it happens.

The Naked Scientists® and Naked Science® are registered trademarks.
Information presented on this website is the opinion of the individual contributors and does not reflect the general views of the administrators, editors, moderators, sponsors, Cambridge University or the public at large.